Synchronous switch circuit for reed relays



July 22, 19.69 R. A. KORN 3,457,432 lSYNCXRONOU?) SWITCH CIRCUIT FORREED RELAYS Filed Nov. 5, 1965 zal/m: y zama United States Patent C)3,457,432 SYNCHRONOUS SWITCH CIRCUIT FR REED RELAYS Roger A. Korn,Washington County, reg., assigner to Allis-Chalmers ManufacturingCompany, Milwaukee,

Wis.

Filed Nov. 5, 1965, Ser. No. 506,458 Int. Cl. H01h 33/59 U.S. Cl.307-133 4 Claims ABSTRACT F THE DISCLOSURE This invention relatesgenerally to control circuits for switches. More specifically, thisinvention relates to an electronic circuit for controlling the operationof a relay so that the relay operates near the zero voltage and currentpoints of an alternating current wave.

For many years there have been available on the market small reedrelays, used as dry circuit switches. These relays are very small andhermetically sealed so that they are relatively insensitive toenvironmental changes. The major limitation on the use of this type of arelay has been its relatively low current interrupting capability. As anexample, a relay may be rated as a three amp AC interrupting switch, butit may be capable of carrying as high as 7 or 71/2 amperes steady-statecurrent. It is only when the circuit is opened or closed that the relayis a problem because arcing at that time pits the contacts and may attimes weld the contacts together. If it were not for this problem, therelays could be rated for much higher currents.

Applicants invention -controls the relay in such a manner that it onlycloses the circuit at or near the zero point in an alternating voltagewave and only interrupts the circuit at or near the zero point in analternating current wave. Therefore, when the relay is actuated, it isinterrupting a very small amount of current which it can easily handle.Hence, this same relay can be rated much higher than its previousinterrupting capacity because the conducting material of the relay iscapable of handling three or four times as much current on a continuousbasis as compared to an interrupting basis. Therefore, the limitingfactor of a relay having this kind of a control circuit associatedtherewith is the upper limit of its conducting capability rather thanits interrupting capability.

Zero current interrupting devices have been used in the past, but werevery complex and expensive. Hence, itwas impractical to use such a zerocurrent interrupting device in combination with small relays. Thisinvention overcomes the above problem by providing a simple triggeringcircuit that can be used to close and open at or near the Zero point ofthe AC voltage wave, respectively.

Therefore, it is the object of this invention to provide a new andimproved control circuit for small relays.

Another object of this invention is to provide ya simple, inexpensiveand reliable control ci-rcuit for initiating the operation of the relayto interrupt the current only at or near current zero.

Another object of this invention is to provide a simple, inexpensive andreliable control circuit for initiating the 3,457,432 Patented July 22,1969 operation of the relay to close the load circuit only at or nearvoltage zero.

Referring more particularly to the drawing by charactersof reference,the control circuit 10 of this invention is illustrated in connectionwith the reed type relay 11 positioned to connect a load 12 directlyacross an AC source 13.The relay control circuit is supplied by a 28volt DC source and has a coil 14 that is inductively related to therelay 11 for closing the relay which is normally biased toward an openposition. The values assigned to the various elements of the controlcircuit are for illustration purposes only and are not intended to belimiting in any way.

'With the movable contact 16 of the relay 11 in engagement with terminal17, the AC line voltage is presented to an appropriate resistor string18 shown as the 500K, 3K, 6.8 Vohm resistor string. The value of theresist-ances is such that only a negligible current fiows through theload and hence this can be considered as a functionally open circuit.This string of resistors 18 acts as an attenuator and provides analternating low voltage sginal in phase with the line voltage. Thissignal is fed to control circuit 10 and is ultimately used to close therelay when the line voltage curve is passing through zero. The value ofthe voltage signal is predetermined by the size of the resistors in thestring and in the illustrated case is a 2 volt peak-to-peak signal. Thesignal is phase shifted by the capacitor 20 and the 30K resistor 21 andis presented to the input 22 of a conventional high gain differenceamplifier 23 that is supplied by an appropriate DC sour-ce illustratedas a 28 volt source. The signal to amplifier 23 is amplifiedsufficiently to drive the output of the amplifier from 28 volts to zerovolts rapidly thus producing a square-wave output which has a zerocrossing phase lead of about 2 degrees.

The output of the difference amplifier 23 is connected to capacitor 25and resistor 26. Here the square-wave output of the amplifier 23 isdifferentiated 'by the capacitor and the resistor-circuit combination togive an output signal having a wave form at the junction 27 that haspositive land negative spikes. This signal is then presented through theclose button 28 and open button 29 to the input terminals 30, 31respectively of the bistable multivibrator 32. Dierence amplifier 23 andbistable multivibrator 32 are known types of apparatus and are describedin detail in a text entitled Pulse and Digital Circuits by Millman andTaub published by McGraw-Hill.

When the close button 28 is depressed, the output of thecapacitor-resistor junction 27 is connected to the input 30. Themultivibrator 32 is designed so that it will receive only positivevoltage pulses through this input 30. Hence, the next positive-goingspike in the signal wave flips the multivibrator 32 to a conductivestate. The output of the multivibrator is connected to a reed relay coil14 which is energized when the vibrator is conducting. This causes thered relay to close and the movable contact 16 of the relay moves fromterminal 17 to terminal 35, thus placing the load 12 directly across theAC line 13. Hence, the circuit is closed.

The multivibrator 32 will remain in a conductive stage until anegative-going spike is presented to terminal 31. This is accomplishedby depressing the open button 29 which connects the output from thedifferentiating resistor-capacitor junction 27 to the input 31 of thebistable multivibrator. The multivibrator 32 is designed so that thisinput terminal 31 accepts only the negative voltage pulses. Hence, thenext negative pulse received at the input terminal 31 will drive themultivibrator 32 to its nonconducting state whether or not the closeswitch 28 is released. This cuts ofi" the current to coil 114, which inturn releases the movable contact 16 of the relay 11 so that it assumesits initial position in contact with terminal 17.

When the relay is energized, the movable contact element 16 of the relayis in contact with terminal 35 and the load is directly across the ACsource. A device such as current transformer 36 monitors the loadcurrent and produces a signal that is proportional to, and in phasewith, the load current and presents it to the control circuit 10. Thissignal is acted on by the control circuit including the differenceamplifier 23 and the resistor-capacitor combinations 20, 21 and 25, 26in substantially the same manner as described above in connection withthe Ysignal produced through the resistor string 18. This results in theproduction of phase-locked spikes at the junction 27 of capacitor 25 andresistor 26. Hence, when the open button 29 is depressed, the nextnegative-going spike in the wave from junction 27 is presented to inputterminal 31 and fiips the multivibrator 32 to its nonconducting stage,thus deenergizing relay coil 14 which causes the movable contact 16 toreturn to its normal position engaging terminal 17.

With the circuit as described above, the relay 11 can close only whenthe AC supply voltage wave passes through zero and can open only whenthe load current wave passes through zero. This protects the relay 11 sothat there is no excessive current or voltage between the contacts whenit is opening or closing and prevents pitting and excessive arcing whichwould burn or cause unnecessary wear on the contacts. Such a relay canbe significantly uprated and still be more reliable and have a longerlife. For example, a reed relay rated at 3 amp capacity was recentlytested with this circuit and performed over million operations at 140volts 7.14 amperes without a failure.

The use of a difference amplifier 23 with a high common mode rejectionratio makes it possible to use an economical, unfiltered 28 volt DCpower supply. In addition, the components of this circuit enclosed inbox 38 are especially adapted to be used as an integrated circuit.Hence, most of this control circuit 10 can be formed into an integratedcircuit and placed in the same enclosure as the relay. This, of course,results in a total package which is only slightly larger than the reedrelay per se and allows the packaging of the relay and substantially theentire circuit Within an appropriate hermetically sealed container. Theresulting relay-control package is relatively insensitive totemperature, pressure, atmosphere and other environmental factors, suchas corrosive and explosive atmospheres, thus providing a much morereliable relay.

Although but one embodiment of this invention has been illustrated anddescribed, it will be apparent to those skilled in the art that variousmodifications and changes can be made therein without departing from thespirit of the invention or the scope of the appended claims.

The embodiments of the invention in which an eX- clusive property orprvilege is claimed are defined as follows:

1. A combination switch and control circuit therefor comprising:

a load connected across an AC power source,

impedance means,

a switch connected between said load and said source, said switch havinga first position connecting said load directly to said source and asecond position connecting said load to said source through saidimpedance means,

means inductively coupled to detect load current and for providing analternating voltage signal in phase with said AC source current whensaid switch is in said position, I

means including said impedance means for providing an alternatingvoltage signal that is an analog of the voltage of said AC source whensaid switch is in said second position,

a control circuit for receiving either of said signals and producing anoutput that has a predetermined time relation to the signal received,

the output of said control circuit being connected to a device formoving said switch between said first and second positions.

2. The combination of claim 1 in which the output of the control circuitprecedes the zero crossing point in the AC source current wave bysubstantially the amount of time required to move said switch from oneof said positions to the other of said positions.

3. In combination:

an AC source,

a load for energization from said AC source,

resistance means,

a switch having one position for connecting said load directly acrosssaid AC source so that said load is energized,

said switch having another position for connecting said load across saidAC source in series with said resistance means so that said load issubstantially deenergized,

rst means inductively coupled to detect load current and for generatingan AC low voltage signal in phase with the load current when said switchis in its one position,

second means including said resistance means for generating an AC lowvoltage signal in phase with the AC source voltage when said switch isin its other position,

phase shifting means for receiving said low voltage signals from saidfirst and second means and for shifting the phase thereof,

a difference amplifier for receiving the phase shifted signal from saidphase shifting means and for producing a square-wave output signal,

differentiating means for receiving said square-wave output signal fromsaid difference amplifier and for producing output signal havingpositive and negative spikes,

a bistable multivibrator for receiving said output signal from saiddifferentiating means and for effecting activation of said switch, and

switching means for selectively presenting either positive or negativespikes of the output signal of said differentiating means to saidmultivibrator to effect actuation of said switch to its one position orto its other position.

4. A control for operating a relay comprising:

an AC source of power,

a load,

a relay normally biased to a first position to substantially deenergizesaid load and having a second position wherein said load is energizedfrom said source,

resistance means connected across said AC source in series with saidrelay and said load when said relay is in said first position, saidresistance means presenting a predetermined voltage signal to a controlcircuit,

said signal being in phase with the AC source voltage,

inductive means coupled to detect load current and to present apredetermined voltage signal to said control circuit when said relay isin said second position and said relay is connected directly in serieswith said load and said AC source,

a DC source of power for said control circuit,

said control circuit comprising a first resistance-capacitor circuitarranged to phase shift said signals,

the output of said first resistance-capacitor circuit being connected toa high gain difference amplifier to transform said signal to asquare-wave, the output of said difference amplifier being connected toa second resistance-capacitor circuit which differentiates saidsquare-wave into a wave having positive and negative-going spikes,

a bistable multivibrator having first and second input terminals, saidiirst terminal lbeing conditioned to accept only positive current pulsesand said second terminal being conditioned to accept only negativecurrent pulses,

means for selectively connecting the output of said secondresistance-capacitor circuit to said rst terminal of said bistablemultivibrator to place it in a conducting mode,

means for selectively connecting the output of said secondresistance-capacitor circuit to said second terminal of saidmultivibrator to place it in a nonconducting mode,

a coil controlling said relay and being connected to the output of saidmultivibrator,

whereby upon connecting said first terminal to the output of said secondresistance-capacitor circuit said multivibrator is forced to itsconducting mode and said coil is energized and moves said relayto itssecond position and upon connecting said second terminal to the outputof said second resistance-capacitor circuit said multivibrator is forcedto its nonconducting state whereby it cuts off the current to `said coiland causes said relay t0 return to its rst position.

References Cited UNITED STATES PATENTS 2,441,789 5/1948 Bivens.3,292,047 12/1966 Lee. 3,309,602 3/1967 Euvino et al 307-133 X FOREIGNPATENTS 96,007 7/1964 Great Britain.

T. B. JOIKE, Assistant Examiner U.S. C1. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3 ,457,432 July 22 1969 Reger A. Korn It is certified that error appears inthe above identified patent and that Said Letters Patent are herebycorrected as shown below:

Column 3, line 70, after "said" insert first Signed and sealed this 21stday of April 1970.

(SEAL) Attest:

Edward M.F1emher,1r. WILLIAM E. SCHUYLER, IR.

Attesting Officer Commissioner 0f Patents

